In some applications of visible light sources, it may be desirable to know when light radiated from the source exceeds a certain brightness. If it is known when the brightness is exceeded, controls such as source regulation or shut-off can be implemented.
Optical applications are becoming increasingly combined with semiconductor control circuits. Also, even some of the optical circuits are capable of being realized with semiconductor fabrication techniques. An example of using semiconductor techniques to fabricate an optoelectrical device is the fabrication of spatial light modulators, especially those known as "deformable mirror devices" or "digital micro-mirror devices". These devices consist of an array of tiny micro-mechanical mirror elements, which are movable between at least two positions and reflect light to an image frame if positioned so that light is reflected in that direction. The mirror elements are controlled and individually addressed with electrodes. The mirror elements, as well as the addressing circuit, can be manufactured on a single semiconductor substrate.
Various types of light detectors are available that can be used to detect light and to generate some sort of signal if the brightness exceeds a certain level. However, it would be advantageous to have such a circuit that could be easily implemented as an integrated circuit, together with other circuitry. If the circuit were simple and small, it could be implemented as a control subcircuit on a larger opto-electrical device.
U.S. Pat. No. 5,179,274, issued Jan. 12, 1993, entitled "Method for Controlling Operation of Optical Systems and Devices" describes a logic circuit that uses two photo transistors to detect a low limit and a high limit of illumination. A need exists for an alternative circuit that uses only one photosensitive device to detect an upper limit.